This invention relates to zeolites that are useful as catalyst supports. In particular, it involves a zeolite Y product with an increased mesopore volume and the process for preparing such zeolite.
Many conversion processes in the petroleum industry are carried out using catalysts having zeolites with the structure of zeolite Y as a major component. In many cases the zeolite Y has been subjected to certain stabilizing and/or dealumination process steps during its preparation that result in the zeolite having a reduced unit cell constant (a.sub.o) and an increased silica to alumina ratio. In general, there are found three classes of these stabilized zeolites Y; the ultrastable zeolites Y ("USY"), the very ultrastable zeolites Y ("USY"), and the superdealuminated ultrastable zeolites Y ("SDUSY"). These stabilized zeolites, as well as the as-synthesized zeolite Y, do not have many pores that are larger than about 2 nanometers (nm) in diameter, such "mesopores" typically having diameters of 2 to 60 nm. Limited mesopore volume within the 2 to 60 nm pore diameter range can be a decided disadvantage when such catalysts are used for processes that tend to coke the catalysts. Examples of such processes are cracking or hydrocracking of heavy crude oils.
The as-synthesized zeolite Y (referred to herein simply as "Y") has a unit cell constant greater than 24.6 up to 24.85 angstroms (.ANG.). These as-synthesized materials have a mesopore volume of less than about 0.05 cc/g. The ultrastable Y zeolite (USY) has a reduced unit cell constant of 24.5 to 24.6.ANG.. These USY type materials have a mesopore volume of less than about 0.17 cc/g. The very ultrastable zeolite Y (VUSY) has a reduced unit cell constant of greater than about 24.27 (approximately 24.3) to less than 24.5.ANG.. The VUSY type materials have a mesopore volume of less than about 0.22 cc/g. The superdealuminated ultrastable zeolite Y (SDUSY) has a reduced unit cell constant of about 24.27.ANG. or less. These SDUSY type materials have a mesopore volume of less than about 0.25 cc/g.
U.S. Pat. Nos. 5,069,890 and 5,087,348 teach a method of preparing a zeolite with the Y structure with a secondary pore volume as high as 0.20 cc/g. These patents teach that secondary pores are 10 to 60 nm in diameter. The method consists of steam calcining previously dealuminated zeolite Y at high temperatures for long periods of time, typically 16 to 24 hours. Since the dealuminated zeolite Y starting material would have been prepared with one or more steam calcinations, at least two steam calcinations are necessary to provide the product with the slight secondary pore volume increase. In the preparation of these products described in the patent examples, the SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the zeolite is increased as a result of the process.
U.S. Pat. No. 5,112,473 teaches a similar small increase in the so-called secondary pore volume by a method involving the acid treatment of dealuminated zeolite Y having a.sub.o measurements of 24.3 to 24.5.ANG..
It is an object of this invention to prepare zeolites with a structure of zeolite Y having increased mesopore volume in pores having diameters of 2 to 60 nm when compared to similar zeolites prepared using conventional processes. It is also an object of this invention to provide such increased mesopore volume by a process not involving calcining with steam.